Organic Light Emitting Diodes (OLEDs) have been known for some years and have been recently used in commercial display devices. Such devices employ both active-matrix and passive-matrix control schemes and can employ a plurality of light-emitting elements. The light-emitting elements are typically arranged in two-dimensional arrays with a row and a column address for each light-emitting element and are driven by a data value associated with each light-emitting element to emit light at a brightness corresponding to the associated data value. However, such displays suffer from a variety of defects that limit the quality of the displays. In particular, OLED displays suffer from non-uniformities in the light-emitting elements. These non-uniformities can be attributed to both the light emitting materials in the display and, for active-matrix displays, to variability in the thin-film transistors used to drive the light emitting elements.
It is known in the prior art to measure the performance of each pixel in a display and then to correct for the performance of the pixel to provide a more uniform output across the display. U.S. Pat. No. 6,081,073 entitled “Matrix Display with Matched Solid-State Pixels” by Salam, granted Jun. 27, 2000 describes a display matrix with a process and control means for reducing brightness variations in the pixels. This patent describes the use of a linear scaling method for each pixel based on a ratio between the brightness of the weakest pixel in the display and the brightness of each pixel. However, this approach will lead to an overall reduction in the dynamic range and brightness of the display and a reduction and variation in the bit depth at which the pixels can be operated.
U.S. Pat. No. 6,473,065 entitled “Methods Of Improving Display Uniformity Of Organic Light Emitting Displays By Calibrating Individual Pixel” by Fan issued Oct. 29, 2002, describes methods of improving the display uniformity of an OLED. In order to improve the display uniformity of an OLED, the display characteristics of all organic-light-emitting-elements are measured, and calibration parameters for each organic-light-emitting-element are obtained from the measured display characteristics of the corresponding organic-light-emitting-element. The calibration parameters of each organic-light-emitting-element are stored in a calibration memory. The technique uses a combination of look-up tables and calculation circuitry to implement uniformity correction. However, the described approaches require either a lookup table providing a complete characterization for each pixel, or extensive computational circuitry within a device controller. This is likely to be expensive and impractical in most applications. In particular, the memory required to store compensation information can be costly. Hence, it is useful to minimize this cost.
One simple technique for compensating AM-OLED displays may be to measure the output of all of the pixels at two pre-determined code values corresponding to presumed luminance output levels. The output can be used to determine a common gain and offset for all of the pixels. However, this technique provides only a global adjustment for the pixels and does not address differences between the pixels. A more complex method is to measure the output of each of the pixels at the same, common pre-determined levels. The output measured for each pixel can be used to provide a custom offset and gain forming a linear approximation of the response of each pixel. However, this second technique may not provide the optimum custom offset and gain since the response of the pixels may not be linear and a linear approximation will therefore create errors at various light levels.
An alternative described in co-pending, commonly assigned patent application U.S. Ser. No. 11/093,115, filed Mar. 29, 2005 by Cok et al., is to measure the output of each pixel at a plurality of levels. The brightness of each light-emitting element at two or more, but fewer than all possible, different input signal values is measured and the measurements employed to estimate a maximum input signal value at which the light-emitting element will not emit more than a predefined minimum brightness (offset) and the rate at which the brightness of the light-emitting element increases above the predefined minimum brightness in response to increases in the value of the input signal (gain). The offset and gain values are used to modify the input signal to a corrected input signal to correct the light output of the light-emitting elements. Such an approach, while useful, still may not minimize the luminance error corresponding to the difference between the desired linear response to a code value and the actual response over the range of code values at which the pixel is operated.
One technique that can minimize the error is to employ a complete look-up table providing a correction for every code value of each pixel. However, such a solution requires a large, expensive memory. Alternatively, a correction curve may be estimated by employing a series of linear correction values defining a series of line segments. Such an approach reduces the memory storage somewhat and may provide approximate corrections but the memory requirements are still large and complex control circuitry may be required to select the appropriate line segment, increasing costs. These approaches are described in co-pending patent application Ser. No. 11/093,115, which is hereby incorporated in its entirety by reference.
There is a need therefore, for an improved method of providing uniformity in an OLED display that overcomes these objections.